The glitch comes at a crucial time for the airline, whose parent company hopes to emerge from bankruptcy by September and complete a merger with US Airways.

American Airlines, hoping to reinvent itself by coming out of bankruptcy with a new logo, freshly painted planes and a merger plan that will make it the nation's largest carrier, suffered a setback Tuesday when a computer outage grounded hundreds of planes across the country.

The computer problem forced the airline to cancel 745 flights, frustrating thousands of passengers who were left to fume and wait at crowded terminals from Los Angeles to Dallas and New York.

Even after the system was restored about two hours later, passengers waited in long lines as airline officials struggled to rebook them on new flights. The airline didn't disclose the cause of the problem but said further cancellations were expected Wednesday.

"It's been a crazy, horrible, miserable day," Karen Winston said as she waited at Los Angeles International Airport before the computer system was back online. The 52-year-old assembly worker was scheduled to fly to Shreveport, La., before her flight was canceled and she was forced to wait in a long line.

A kidney transplant patient, Winston said she was worried that she would have to take her anti-rejection pills on an empty stomach until an airline supervisor brought her a sandwich and water bottle.

"I'm just upset about the whole thing," she said.

At Dallas/Fort Worth International Airport, Malcolm Woods, a 26-year-old software consultant, said he was forced to jump into a car and drive to Austin, Texas, for a technical conference after his flight was canceled.

"I'm on edge," he said. "All of our plans are kind of ruined."

The computer glitch comes at a crucial time for American Airlines, whose parent company, AMR Corp., hopes to emerge from bankruptcy by September and complete a merger with US Airways to become the nation's largest airline.

American recently adopted a new logo and a modern red, white and blue livery. The carrier also plans to take delivery over the next few years of 460 new narrow-body jets, plus 20 Boeing 777-300ER planes with lie-flat seats in first- and business-class sections, mood lighting and a walk-up bar.

The airline has begun airing several commercials in which the narrator says, "We're building the youngest, most modern fleet among the largest U.S. airlines." The ads end with the tag line: "We are becoming the new American."

If American rebooks most of its passengers without further delays or cancellations, airline analysts say, the problems won't cause long-term damage to the airline's image.

"Right now, the damage seems to be minimal," said Joe Brancatelli, a business travel expert who writes a regular travel column on http://www.joesentme.com. "But there is no good that came from this."

American quickly announced that it would waive change fees for delayed passengers who want to rebook. For passengers who don't want to reschedule, American said it would offer full refunds.

If more computer problems arise in the future, particularly as American attempts to merge its computer system with that of US Airways, the airline could lose many passengers. United Airlines had several computer outages last year as the carrier tried to complete a merger with Continental Airlines.

"It's not a good thing and I'm sure they wish it wouldn't happen," Seth Kaplan, a managing partner at trade publication Airlines Weekly, said about American's computer problems. "But airlines have bounced back from even more serious and unfortunate things than this."